: That's pretty much how Steam works. You have to log on after a certain period
: of time (like, weeks) to re-authenticate, but elsewise, you can run
: offline mode simultaneously on any number of machines. Valve doesn't seem
: to be hurting too badly for that, though.

You can do that with Steam, and only slightly less conveniently (logging in), but relatively few people do thanks to the subscriber agreement being very blatant about account sharing.

Such moral and/or account-risking concerns would quite possibly be less of a consideration if everyone is using their own accounts and operating in a way which would be substantially more legitimatesque. "I was just playing offline" feels more justified than "I'm prohibited from using this account but I'm using it anyway." Without a time limit for offline play, they'd pretty much have to legitimize it with at most a guideline along the lines of "please don't abuse the system by intentionally playing while someone else is." The abuse wouldn't be all that much easier, but it also wouldn't be as clear a violation of terms.